Tungsten filament lamps have been heavily relied on in the past for navigational signal lighting. In order to produce red light, the tungsten filament lamp may be surrounded by a red-colored filter or the lamp envelope may be colored with a red dye. Besides having low efficiencies, the filament in such lamps is very brittle and therefore susceptible to shock and vibration. This results in premature lamp failure. Also, in general, they have short life of about 500 hours.
Modern light sources, more particularly arc discharge sources, have been or are being developed for navigational signal lighting applications because of the many advantages offered by these light sources. It is well known that an arc discharge source generally provides better efficacy and longer life than its tungsten filament lamp counterpart. Since the electrodes are heavier than the filament, the lamp may be more rugged and less susceptible to shock and vibration.
In an arc discharge lamp, the length and width of the arc are design variables to a large extent. In an tungsten filament lamp, the length and width of the filament are for the most part determined by the lamp wattage. Thus, there is greater flexibility in the choice of optical characteristics of the light source with arc discharge lamps than with comparable tungsten filament lamps.
The principal object of a navigational signal light is to emit as much light flux as possible from a reliable light source and direct the light into the plane of the horizon. The light may be collected into one or more narrow beams which are mechanically rotated, or it may be radiated in all horizontal directions simultaneously. There are basically two types of rotating beams or beacons. In the first type, a reflector or other means of concentrating the light is used with the lamp. The entire optical system is rotated. This method generally produces a single beam; all of the emitted light is swept through 360 degrees. For an example of this first type of beacon and an arc discharge lamp for use therewith, see U.S. Pat. No. 4,847,530 which issued to English et al and which is assigned to the same Assignee as the present application. This patent describes an arc discharge lamp which, in one specific example, is rated at 175 watts.
In the second type, a rotating screen surrounds a stationary lamp. The screen contains multiple lenses or other means for concentrating light. The rotating screen generally produces multiple rotating beams, one beam associated with each lens or sector subtended by a lens. The emitted light within any sector is formed into a pencil beam and swept only within that sector. It is this type of beacon and, more particularly, the light source associated therewith, which is the subject of this disclosure.
For an example of this second type of beacon and an arc discharge lamp for use therewith, see U.S. Pat. No. 4,864,180 which issued to English et al and which is assigned to the same Assignee as the present application. This patent describes a metal-halide arc discharge lamp which, in one specific example, is rated at 45 watts.
The arc discharge lamps in the above-described patents are quite suitable for various navigational lighting applications wherein a "white" light is desired. Even if these lamps are used with red-colored filters, they would still have the disadvantage of generating too much heat if enclosed within the relatively small beacon enclosure of a typical lighted buoy. Further, the percentage of the visible spectrum that is red is less for conventional white arc discharge lamps than for incandescent lamps.
Other sources of red light other than the examples discussed above are also available. For example, fluorescent lamps which emit radiation primarily in the red region of the spectrum are well known in the art. U.S. Pat. No. 3,365,232, which issued to Repsher on Jan. 23, 1968, discloses a fluorescent lamp having a phosphor coating which emits mainly in the red region of the spectrum in response to ultraviolet radiation in combination with an underlying layer of red pigment that filters out all of the visible radiation below a certain wavelength (e.g., 600 nanometers). Although this lamp is effective in producing a red component of light, the red is not saturated. The light form such a fluorescent lamp tends toward pink due to the presence of violet, blue, green, and yellow lines of mercury radiation from the arc itself. A saturated red can be produced by adding filters to the source, but this can cause a considerable loss of efficiency.
Another source of red light is conventional low pressure, neon arc discharge lamps. This light source, which consists of long tubes and is generally used for advertising signs, is filled with pure neon at a relatively low pressure. The form of the tube is seldom straight and is usually tailor-made to the customer's wishes. Although the neon sign is also effective in producing a reddish light, the light is more orange than red. Filtering to remove the shorter wavelengths is required to meet specifications for signal red, and this reduces efficiency. Due to the low pressure filling of the lamps (on the order of 10 torr), they are relatively large in size to achieve good life, efficiency, and maintenance. More important, the light radiated per unit area of tube wall is low. The large source size necessary to produce the required light and the low radiation density are the converse of properties required for the optics of navigational beacons and similar devices.
It would be an advancement of the art if a light source could be provided which is suitable for use in a navigational signal light, such as a lighted buoy.